The biosynthesis of amino acids in plants is highly regulated (Lea et al., 1985, The Biosynthesis of Amino Acids in Plant, In: Chemistry and Biochemistry of the Amino Acids, G. C. Barrett, ed., Chapman & Hill, London, pp. 197-226). Due to the general desirability for essential amino acids in grains and other foods, several approaches have been developed to enhance the content of amino acids in plants (see e.g. G. Galili and B. A. Larkins, 1999, Enhancing the Content of Essential Amino Acids Lysine and Threonine in Plants, In: Plant Amino Acids in Biochemistry and Biotechnology, B. K. Singh, ed., Marcel Dekker, Inc., New York, pp. 487-507). Also, in tobacco, certain amino acids can be a major source for flavor and aroma production.
However, amino acids undergo the Maillard reaction and Stecker Degradations in tobacco and produce pyrazines and other nitrogen containing compounds, some of which may not be beneficial. The amino acid profile affects not only the yield, but also the type, of pyrazines formed (Lu, G., et. al., 1997, J. Agric. Food Chem., 45: 233-236; J. Chen and C.-T. Ho, 1999, J. Agric. Food Chem., 47: 643-647). Thus, although there has been a focus on increasing amino acid levels in tobacco, there may also be a need to produce tobacco plants, and tobacco products derived from such plants, having reduced amounts of certain amino acids. Certain amino acids may be associated with the production of potentially harmful chemicals upon heating and/or burning the tobacco. For example, tobacco smoke may include acrylamide, which has been characterized as a Group 2A carcinogen by the International Agency for Research on Cancer (IARC). Also, acrylonitrile is a IARC Group B1 carcinogen found in cigarette smoke.
Thus, there is a general need for methods that can provide plant lines comprising reduced biosynthesis of particular amino acids. The methods should be designed so that they can be used even for plant species such as tobacco that are large so as to require extensive facilities for breeding, and that have a complex genome and thus can require screening of a large number of mutation events to isolate the mutation of interest.